Speckle shearography dates back to the early 1970s. An electronic version of speckle shearography, ES, was developed in 1980 and, since that time, ES has been applied to the study of many different phenomena, primarily in research laboratories. Such applications range from the nondestructive inspection of aircraft to vibration analysis, materials characterization, and electronic packaging study. ES has also been used to locate subsurface disbonds between silicone rubber and the substrate of missiles, to detect disbonds in aircraft fuselage panels, to inspect airskin structures, and in Q-switched pulsed lasers (which are complex and expensive).
The NDI (nondestructive inspection/evaluation) methods most widely used in industry are ultrasonics, eddy-current measurement, and x-radiography. The ultrasonic techniques are used to detect flaws by measuring the response to an ultrasonic stress wave. However, due to the point-by-point or line-by-line scanning procedures involved, the ultrasonic method is typically slow. A medium, such as water or gel, is usually required to transfer the ultrasound energy from the transducer into the material, which is inconvenient in some cases.
X-radiography relies on the differential absorption or scattering of x-ray photons as they pass through a material. Flaws that either allow more x-ray photons to pass or that absorb or scatter the photons can be imaged if the effect is sufficiently pronounced. The molecules in many polymer composites are usually of low atomic weight nuclei, and hence the absorption of x-rays is low and contrast is usually poor, especially for a thin plate. Eddy-current measurement is only applicable to metallic materials. Furthermore, none of these methods relate flaw detection to the stress/strain states of a test object in any fundamental way. Therefore, how a detected flaw affects the performance of a particular component cannot be revealed by current detection processes.
Unlike these traditional methods, loading (stressing) is an essential part of NDI processes based upon ES. The loading provides an important connection between the detected flaws and the effect of the flaws on the integrity and strength of the structures under test.
Ultrasonic and x-ray technologies are good at determining the geometry and detailed location of the flaws, especially the internal flaws in a structure. Unfortunately, neither of these methods relate the detection to the effect of the flaws on material integrity or strength. Furthermore, for inspection of a large area, these techniques are slow and costly. Although ES is most sensitive to surface and subsurface flaws, internal flaws can also be detected from their induced "disturbances" on the surface when an appropriate stressing technique is used.
Among all NDI techniques discussed, laser interferometry, including holographic interferometry and ES, are the only tools which can detect flaws through the direct measurement of a material's strength-related parameters such as deformation/displacement or strain. These techniques also offer the opportunity for directly assessing the actual effects of the detected damage on the structures. Laser interferometry techniques are highly sensitive to a wide variety of flaws and can inspect a large area at fast speed.